In a typical papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper “stock”) is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rolls. The belt, often referred to as a “forming fabric,” provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface of the upper run (i.e., the “machine side”) of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more presses (often roller presses) covered with another fabric, typically referred to as a “press felt.” Pressure from the presses removes additional moisture from the web; the moisture removal is often enhanced by the presence of a “batt” layer of the press felt. The paper is then transferred to a dryer section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Cylindrical rolls are typically utilized in different sections of a papermaking machine, such as the press section. Such rolls reside and operate in demanding environments in which they can be exposed to high dynamic loads and temperatures and aggressive or corrosive chemical agents. As an example, in a typical paper mill, rolls are used not only for transporting the fibrous web sheet between processing stations, but also, in the case of press section and calender rolls, for processing the web sheet itself into paper.
Typically rolls used in papermaking are constructed with the location within the papermaking machine in mind, as rolls residing in different positions within the papermaking machines are required to perform different functions. Because papermaking rolls can have many different performance demands, and because replacing an entire metallic roll can be quite expensive, many papermaking rolls include a polymeric cover that surrounds the circumferential surface of a typically metallic core. By varying the material employed in the cover, the cover designer can provide the roll with different performance characteristics as the papermaking application demands. Also, repairing, regrinding or replacing a cover over a metallic roll can be considerably less expensive than the replacement of an entire metallic roll. Exemplary polymeric materials for covers include natural rubber, synthetic rubbers such as neoprene, styrene-butadiene (SBR), nitrile rubber, chlorosulfonated polyethylene (“CSPE”—also known under the trade name HYPALON from DuPont), EDPM (the name given to an ethylene-propylene terpolymer formed of ethylene-propylene diene monomer), polyurethane, thermoset composites, and thermoplastic composites.
In many instances, the roll cover will include at least two distinct layers: a base layer that overlies the core and provides a bond thereto; and a topstock layer that overlies and bonds to the base layer and serves the outer surface of the roll (some rolls will also include an intermediate “tie-in” layer sandwiched by the base and top stock layers). The layers for these materials are typically selected to provide the cover with a prescribed set of physical properties for operation. These can include the requisite strength, elastic modulus, and resistance to elevated temperature, water and harsh chemicals to withstand the papermaking environment. In addition, covers are typically designed to have a predetermined surface hardness that is appropriate for the process they are to perform, and they typically require that the paper sheet “release” from the cover without damage to the paper sheet. Also, in order to be economical, the cover should be abrasion- and wear-resistant.
As the paper web is conveyed through a papermaking machine, it can be very important to understand the pressure profile experienced by the paper web. Variations in pressure can impact the amount of water drained from the web, which can affect the ultimate sheet moisture content, thickness, and other properties. The magnitude of pressure applied with a roll can, therefore, impact the quality of paper produced with the paper machine.
Other properties of a roll can also be important. For example, the stress and strain experienced by the roll cover in the cross machine direction can provide information about the durability and dimensional stability of the cover. In addition, the temperature profile of the roll can assist in identifying potential problem areas of the cover.
It is known to include pressure and/or temperature sensors in the cover of an industrial roll. For example, U.S. Pat. No. 5,699,729 to Moschel et al. describes a roll with a helically-disposed leads that includes a plurality of pressure sensors embedded in the polymeric cover of the roll. The sensors are helically disposed in order to provide pressure readings at different axial locations along the length of the roll. Typically the sensors are connected by a signal carrying member that transmits sensor signals to a processor that processes the signals and provides pressure and position information.
More particularly, as each sensor passes through a nip, the sensor becomes loaded and emits a signal. The sensor then becomes unloaded after it passes through the nip. However, the sensors are serially connected by the signal carrying member, and sensor signals can overlap or superimpose if more than one sensor is passing through a nip at the same time. Accordingly, the system may not produce an accurate pressure profile in certain applications.
The sensor signals can overlap in extended or wide nip applications. For example, an industrial roll can be positioned relative to a mating structure, such as a shoe of a shoe press, to form a relatively wide nip therewith. In this instance, at least adjacent sensors can be located in the nip at the same time, and this can result in erroneous measurements.
Signals can also overlap or be superimposed in applications in which a roll is positioned so as to mate with multiple mating structures, thereby creating multiple nips. Exemplary applications include grouped rolls in a press section and rolls in a calendering section. In these instances, at least one sensor can be in each nip at a particular time. Again, this can result in erroneous measurements.